It is known to recover heat of exhaust gas exhausted from an exhaust system of a vehicular engine based on the principle of heat pipe and use the recovered heat for other purposes such as for warming up the engine. As an example of an exhaust heat recovery apparatus, Japanese Unexamined Patent Application Publication No. 4-45393 describes a looped heat pipe heat exchanger.
The disclosed heat exchanger includes a closed looped circulation passage filled with an internal heat-transfer fluid, an evaporation unit disposed on the circulation passage for evaporating the internal heat-transfer fluid therein by receiving external heat and a condensation unit disposed on the circulation passage at a position higher than the evaporation unit for performing heat exchange between the evaporated internal heat-transfer fluid and an external heat-transfer fluid.
In such a heat exchanger, efficiency is improved by increasing the surface area for boiling the internal heat-transfer fluid in an evaporation unit. To increase the boiling surface area, for example, it is proposed to form grooves on inner surfaces of tubes of the evaporation unit by cutting.
In fact, the evaporation unit is subjected to a high temperature environment. Therefore, the tubes of the evaporation unit need to have strength and to be resistant to oxidizing. To form the grooves on the inner surfaces of the tubes by cutting, it is necessary to increase the thickness of tube walls. The increase of the thickness of the tube walls results in an increase of weight and a reduction of yields. Further, in the case that the grooves are formed on the inner surfaces of the tubes by cutting, the tube walls need to have sufficient thickness in consideration of stress applied thereto.